Infra-red heaters are used in equipment for treating substrates such as in the drying of paper. A particularly effective burner is described in U.S. Pat. No. 4,722,681. As shown and described therein, the burner body has a plenum chamber divided by a baffle into an unbaffled upstream intake compartment and a baffled downstream intake compartment. A matrix is located at the downstream end of the downstream intake compartment. The matrix is disclosed as being made from ceramic fibers about one inch thick and is adhesively secured to the side walls of the burner body. The matrix is formed as a block wherein its side walls are perpendicular to its top and bottom walls. The matrix fits against the comparably shaped end portions of the side walls of the burner body.